SOMERVILLE — How ugly is the McCarthy Overpass, the elevated highway dividing Union Square and East Somerville? Its steel is rusted and flaking, its concrete pockmarked and crumbling. The underside is a lunar landscape of concrete dust, litter, and pigeon droppings.

“It just repels you,” said Hayes Morrison, Somerville’s director of transportation and infrastructure, shuddering at the thought of walking beneath the eroding span, which carries the McGrath Highway (Route 28) over surface streets.

Of course, the 1950s planners who built the overpass paid little heed to the people who might walk, bike, or reside in its shadow. Urban neighborhoods were meant to be leapfrogged by suburban commuters; road builders fixated on blasting away bottlenecks.

Now that the McCarthy is falling apart, even the state Depart­ment of Transportation agrees it is an overbuilt vestige and has promised to take it down. But the contractors who mobilized beneath it recently are not there to dismantle it. ­Instead, the state is reinforcing the McCarthy, spending $10.9 million to keep it standing for a decade or more.

Transportation officials say that will buy them the time needed to develop replacement plans and secure $70 million for a network of landscaped surface streets and sidewalks, with crosswalks, improved inter­sections, well-timed traffic lights, and bicycle lanes.

Advocates for removing overpasses call it throwing good money after bad. But the state’s top highway official said it would be a mistake to remove the McCarthy without being ready to follow up with re­designed surface roads.

“It’s not as simple as just remov­ing the overpass,” said Frank DePaola, the Department of Transportation’s highway ­administrator. “We’d end up with much worse traffic congestion.”

‘It’s not as simple as just removing the overpass. We’d end up with much worse traffic congestion.’

A similar course is planned next year with the Bowker Overpass, the elevated highway that splits the Back Bay from Kenmore Square and casts a shadow over Frederick Law ­Olmsted’s Emerald Necklace. More fraught than the ­McCarthy, the Bowker connects Storrow Drive with the Fenway and Longwood Medical Area, and spans the Massachusetts Turnpike and railroad tracks, as well as the park and the neighborhood.

So the state will invest $10 million in that structurally deficient span, as well, buying time to analyze traffic and contemplate whether to rebuild the Bowker in part or in full.

Those who want to see the McCarthy and Bowker come down worry the repairs will ­allow the state to drag its feet on more substantial work.

“We want to see these projects move to ‘shovel ready’ and not just shored up and then left behind,” said Jackie Douglas, executive director of the ­LivableStreets Alliance
, a nonprofit that promotes walking, biking, and mass transit.

Winning widespread support for such projects can be challenging, judging from two other campaigns to address decay­ing vestiges of mid-20th century planning.

Consider Jamaica Plain’s Casey Overpass, which channels the Arborway over Forest Hills. In March, the state said it would raze the overpass and ­replace it with surface streets, following months of meetings with a local committee. Neighbors who believe the community is being railroaded have vowed to fight the project.

Each case is laced by competing undercurrents that have long coursed through local history, with Boston both a seat of innovation and a place resistant to change.

Forty years ago, the region was a pioneer in rejecting highways as the best way to revitalize stagnating cities. Governor Francis W. Sargent disavowed his prohighway past and rejected the Southwest Expressway and the Inner Belt. The Southwest Expressway would have barreled an interstate straight through Jamaica Plain and Roxbury to downtown Boston; the Inner Belt would have cut through Roxbury, the Fens, Brookline, Cambridge, and Somerville with an eight-lane highway.

Sargent also canceled plans for other urban highways and called for massive investment in transit and commuter rail ­instead.

“It was the gutsiest decision a governor ever made,” said former governor Michael Dukakis, a Democrat who led the pro-city movement as a Brookline legislator and a candidate for statewide office in the late 1960s and early ’70s. The success, if not fully realized, kept Boston from succumbing to sprawl, preserving and promoting the human scale, character, and transit-oriented design now prized by many residents, developers, and tourists.

Now, the expiration of the car-centric infrastructure that preceded Sargent’s shift — the McCarthy, the Casey, the Bowker, and Rutherford Avenue — provides an opportunity to continue the work, Dukakis said.

“Overpasses — get rid of them,” he said.

But canceling plans for new highways is easier than undoing built ones, even if they were often constructed to handle sky-high traffic predictions that either never materialized or that shifted to the Big Dig or MBTA lines that did not exist at the time.

Removing the overpasses ­requires a leap of faith on the part of the public, to accept that there can be more than one way to manage traffic, said Pete Stidman, executive director of the Boston Cyclists Union
, an advocacy group. “After the Casey’s done” — the state has money and a four-year timeline for it — “people will witness how Forest Hills changes and momentum will build for taking down the others,” he said.

Overpass removal would be a bolder gesture than adding bike lanes on streets, incorporating more space for walkers and bicyclists on the rebuilt Charles River bridges, or even making room for bike racks or miniparks by removing some parking spaces. But advocates see all of these moves as a modest rebalancing, with considerable public space still devoted to driving and parking cars. Others see an onslaught against the automobile.

“It’s amazing to me that aesthetics or one transportation mode seems to have captured the Commonwealth’s pocketbook,” the Somerville Democrat said. “I’m a biker, and I’m very supportive of making the city more biker friendly, but I’m also a driver, and 90 percent of my constituents still are, and I have bought in a long time ago to the concept of sharing the road, not taking the road away.”

Capuano said the state must do a better job of regional coordination, especially given the ­financial constraints. Removing the McCarthy could be a mistake, he said, without the long-awaited Green Line extension through Somerville or further widening of Interstate 93.

“If you think people are going to stop driving tomorrow just because you make it more difficult, I guess they haven’t seen the stories of LA, Washington, D.C., and other cities,” Capuano said.

But John O. Norquist, president of the international Congress for the New Urbanism
, which promotes a return to the urban design of the prehighway era, said such projects have overwhelmingly succeeded in places as varied as Seoul and Milwaukee, New York City, and Portland, Ore.

“In America there’s [often] this outdated, outmoded attitude that cities are obstacles that you have to blast giant traffic machines through, and even in a sophisticated place like Massachusetts, your MassDOT, they still often don’t get it,” said Norquist, a former Milwaukee mayor. “If they eliminate grade separation” — meaning underpasses, overpasses, and roads that resist bikes and pedestrians — “everybody will be happy with it after they’re done, and any predictions of traffic armageddon will not be [realized].”